US4152786A - Compensator for implanted blood pump - Google Patents

Abstract

A compensator designed to be implanted within a living body and connected to an implanted reciprocating blood pump. A housing defines a gas-tight chamber which is separated into front and rear portions by a flexible membrane. A piston attached to the membrane guides the movement of the membrane, and a spring mechanism which links the piston and the housing is designed to maintain substantially constant pressure in the front chamber throughout the discharge and fill strokes of the reciprocating blood pump. Two pivoted, diametrically opposed, compression springs are compressed at rest at the end of the fill stroke and exert a force on the membrane which overcomes the differential pressure acting in the opposite direction when the membrane moves so as to expand the rear chamber volume.

Description

This invention relates to implantable heart assist devices and more particularly to a pressure-compensating accessory for use in conjunction with a reciprocating blood pump.

A reciprocating blood pump, as for example one which is designed as a left ventricular assist device, creates a variable volume as a result of its inherent reciprocating movement. If the rear or non-blood side of the pump is sealed off completely, the volume changes result in a large pressure fluctuation in the sealed-off space which impedes proper pump operation.

Various compensation schemes have been proposed for alleviating the undesired affects of such pressure fluctuations including, for example, percutaneous venting, saturated vapor compensation and flexible chamber walls. Percutaneous venting is considered to be the simplest technical solution; however, clinically, there are problems associated with possible vent obstrution, noise, as well as a potential for infection. While a saturated vapor compensation system appears feasible conceptually, it appears that the heat-transfer mechanism required to maintain saturated vapor at constant temperature during expansion and compression is difficult to achieve. The possibility of failure through fatigue has deterred the use of a flexible wall arrangement.

The present invention provides a constant pressure compensator which is designed to be small and compact in size, to compensate for the variable volume on the rear side of a reciprocating blood pump. The compensator uses a compression spring mechanism in combination with a flexible diaphragm to maintain a substantially constant pressure on the rear side of the pump to assure proper pump operation.

A thorough understanding of the invention will result from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings wherein:

FIG. 1 is an enlarged sectional view showing a compensator in the operating position it assumes when the connected blood pump has just completed the fill or intake stroke;

FIG. 2 is a view similar to FIG. 1 showing the compensator in the position it assumes when the pump has just completed its discharge stroke;

FIG. 3 is a view, reduced in size, showing the compensator connected to a left ventricular assists blood pump; and

FIG. 4 is a graph illustrating the spring force vs. the displacement.

The variable volume compensator includes ahousing 11 having a generally circular cross section which is defned by acylindrical sidewall 13 and which is closed by an integralrear wall 15. Afront portion 17 of the housing is separately formed and provided with atubular extension 19 which is connected by a pressure-resistant line 21 to a reciprocating circulation device orblood pump 23, which in the illustrated embodiment is a left ventricular assist blood pump. The connection of theline 21 is such that it is in fluid communication with thepump 23 at a location at the rear of the pump pusher plate which reciprocates to alternately fill a pumping chamber with blood and then discharge such blood.

The housing portions are made of a rigid material which is biocompatible, for example, a thick-walled polymeric material, such as a polycarbonate, or an appropriate metal, such as stainless steel or vitallium. Thehousing 11 may also be covered with a suitable coating to increase its biocompatibility. Thehousing 11 provides a gas-tight, rigid vessel which is divided into afront chamber 25 and arear chamber 27 by adiaphragm 29. Thediaphragm 29 is a flexible membrane of a type well known in the art and is generally circular in outline. The diaphragm may be made from a synthetic rubber or elastomeric material, such as that sold under the trademark Viton, and may be molded to the desired shape. Diaphragms of this general type have been used in diaphragm pumps for pumping various fluids as well as for creating vacuums. See for example, U.S. Pat. No. 3,021,792, issued Feb. 20, 1962, which discloses a fuel pump of the diapragm type.

Thehousing 11 is made in two pieces as illustrated, and thefront portion 17 is suitably mated to the edge of thesidewall 13 of the main portion. The circular perimeter of thediaphragm 29 is suitably clamped between the two parts of thehousing 11, as diagrammatically illustrated in the drawings.

The rear surface of thediaphragm 29 is attached to and reinforced by apiston 33 which has a circular front face that is disposed centrally of the membrane. Thepiston 33 includes a rearwardly extendingrod portion 35 which terminates just short of therear wall 15 of the housing. The piston may be attached to the rear surface of the diaphragm by cementing or by pinning or by any other suitable means, as is also well known in the art exemplified by the aforementioned patent. Thepiston 33 is preferably formed with a rearwardly extendingskirt 37 which tends to guide the portion of the diaphragm that lies outward of the piston front face so that a rolling motion of the diaphragm is created during the motion of the piston (compare FIGS. 1 and 2). Thediaphragm 29 itself will tend to stabilize the path of the front of thepiston 33 during its movement and the rear end is linked to the housing as described hereinafter. However, if desired, some additional slide bearing arrangement (not shown) could be located generally in the location interior of thepiston skirt 37 to assure smooth movement.

Theextension tube portion 19 of the housing is in fluid communication with the rear side of the reciprocatingblood pump 23. Accordingly, as the blood pump works on its discharge or pumping stroke, the space adjacent the rear side of the pumping pusher plate will increase in volume, thus tending to create a low pressure region in the extension tube and thefront chamber 25 of the compensator. Oppositely, when the reciprocatingpump 23 is working on the intake or fill stroke, the volume of this space will decrease and thus tend to create a higher pressure zone. These tendencies are compensated for by the movement of thediaphragm 29. It is the purpose of thecompensator 11 to provide a diaphragm which moves forward and backward so as to maintain the pressure in theforward chamber 25 substantially constant, i.e., within about ±10 Torr. depending on the size of the housing. To accomplish this objective, thecompensator 11 includes aspring mechanism 41 which produces a force which is substantially equal and opposite to the force resulting from the differential pressure which is created as a result of the movement of the pusher plate of thereciprocating blood pump 23.

Thehousing 11 is provided with a pair of inwardly extending and diametrically opposedears 43 which are formed with a central aperture to receive apivot pin 45. Connected to thepivot pins 45 are the outward ends of a pair ofspring retainers 47, and disposed in each of thespring retainers 47 is acompression spring 49. The inward ends of thespring retainers 47 are pivotally mounted bypins 51 to diametrically opposed locations on thepiston rod 35 near the rear end thereof. The arrangement of thesprings 49 is such that they guide the movement of the rear end of thepiston rod 35 as it travels back and forth within therear chamber 27.

In the position shown in FIG. 1, thecompression springs 49 are loaded and are exerting a force. However, because thesprings 49 are here coaxial and diametrically opposed, the forces exerted by both springs cancel each other out. As earlier indicated, when thepiston 33 is in this rearward or withdrawn position, theblood pump 23 has just ended its fill stroke. Preferably, the construction of the device is such that, at this point in time, the pressure within both therear chamber 27 and thefront chamber 25 will be equal to each other and about one atmosphere.

As the pusher plate of thereciprocating blood pump 23 begins its discharge stroke, the volume of the space adjacent the rear side of the plate increases thus decreasing the pressure within thefront chamber 25. As a result, the slightly higher pressure within therear chamber 25 causes the diaphragm to begin to move to the right, as depicted in FIGS. 1 and 2. As soon as thespring mechanism 41 is offset from its dead-center or at-rest position shown in FIG. 1, the pair ofcompression spring 49 begin to exert a force in the direction (based upon the standard "xy" coordinates). The spring constant of thecompression springs 49 is chosen to be such that the force applied by the uncoiling compression spring increases as they expand to substantially balance out the effect of the differential pressure resulting from the partial vacuum which is being created in therear chamber 27. This is done by operating in the range wherein the springs have a negative spring constant. See for example FIG. 4. Accordingly, thepiston 33 and thediaphragm 29 move smoothly to the right until the position as shown in FIG. 2 is reached. As a result, the pressure within thefront chamber 25 remains substantially constant at about 1 atmosphere.

When the pumping stroke of the pusher plate of theblood pump 23 has ended and the fill or intake stroke begins, the pressure within thefront chamber 25 will tend to increase and cause thediaphragm 29 andpiston 33 to move back to the left. The original slight increase in pressure creates sufficient differential pressure relative to the substantially lower than atmospheric pressure in the expandedrear chamber 27 to overcome the force of thecompression springs 49; and thus, thepiston 33 moves to the left compressing thecompression springs 49 until the atrest position is again reached that is depicted in FIG. 1. In this position, the pressure in therear chamber 27 has returned to its original pressure, i.e., about 1 atmosphere, and throughout the intake stroke of the pump, the movement of thediaphragm 29 has caused the pressure in the forward chamber to remain very close to a constant value, i.e., 1 atmosphere. By optimizing thespring mechanism 41, it has been found that a net force approaching zero over the entire operating range can be achieved. By keeping the mass of the moving parts low, so as to minimize the dynamic inertial effects, it is believed that a net force on thepiston 33 of ± about 5 Torr. can be obtained, in anoverall housing 11 volume of about 150 cc., and thus a net pressure fluctuation of this magnitude in the pressure on the rear of the pump pusher plate.

It is believed that this compensator offers a compact, practical approach to the problem of variable volume that is created as a result of an implanted reciprocating blood pump. Because the housing is made of a rigid material, it can be encapsulated conventionally to provide biocompatibility, or as indicated hereinbefore, it can be made from a biocompatible, rigid material (e.g., titanium, stainless steel, vitallium, or polycarbonate). Moreover, because theflexible diaphragm 29 is totally encapsulated within the outerrigid housing 11, its biocompatibility does not come into consideration.

Although, preferably the pressures in thechambers 25 and 27 are equal, thecompensator 11 could be biased to assist the filling stroke of thepump 23. This can be accomplished by setting the pressure in therear chamber 27 so that there will always be a constant small force acting in the left-hand direction so as to assist the fill stroke of the blood pump. In such an instance, the pressure within therear chamber 27 could be set slightly below 1 atmosphere, in the position as shown in FIG. 1, while the pressure in the front chamber is about 1 atmosphere.

Although the invention has been described with regard to a certain preferred embodiment, various changes and modifications as would be obvious to one having the ordinary skill in the art may be made without departing from the scope of the invention which is defined solely by the claims appended hereto.

Various of the features of the invention are set forth in the claims which follow.

Claims (8)

What is claimed is:

1. A compensator designed for implantation within a living body in connection with an implanted reciprocating blood circulation device which compensator comprises

a housing which defines a gas-tight chamber,

a flexible membrane mounted in said housing which divides said chamber into a front portion and a gas-tight rear portion, said membrane being designed to permit movement to change the volume of said rear chamber portion,

means for connecting said front portion in fluid communication with the reciprocating circulation device,

compression spring means mounted in said rear portion, and

means linking said spring means to said flexible membrane so that said compression spring means exerts a force against said membrane during the pumping stroke of said blood circulation device, which force assists in the movement of said membrane in a direction which causes the volume of said rear chamber portion to expand.

2. A compensator in accordance with claim 1 wherein said spring means comprises a pair of compression springs which are pivotally mounted to said housing in locations so as to be generally coaxial and opposed to each other at the end of the intake stroke of the reciprocating device when said rear chamber portion is at its least volume.

3. A compensator in accordance with claim 2 wherein piston means is connected to the rear surface of said flexible membrane and to each of said compression springs.

4. A compensator in accordance with claim 3 wherein each of said compression springs is pivotally connected to a rear location on said piston means with said pivot point being generally aligned with said common axis at the end of said intake stroke.

5. A compensator in accordance with claim 4 wherein both said springs are compressed at the end of said intake stroke and the force profile of said springs is such that the net force on said piston-membrane combination is not more than about ±5 Torr. when the volume of said housing is about 150 cc. or less.

6. A compensator in accordance with claim 1 wherein the pressure within said front and rear chamber portions is about equal at the end of the intake stroke.

7. A compensator in accordance with claim 1 wherein the pressure within said rear chamber is less than that within said front chamber portion at the end of the intake stroke so that said compensator assists the intake stroke of the reciprocating device.

8. A compensator in accordance with claim 1 wherein said spring means have a negative spring coefficient in their operating range.

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